[b]dual[/b] stepper motor system

i'm trying to buil a 2D plotter/printer that can draw and cut into sheets of metal or such... and i'm thinking of using 2 steppers... one for X axis movement and one for Y axis movement....
I hope the parallel port is enogh for that, because I already know how to program a stepper via it... I used an old stepper from a printer (7.5 degrees per step, 5 wires, 4 data pins in the port) I can control how many steps I want the motor to take and the speed...
my big problem is: How do I syncronize the two steppers so they can draw any picture I want (in 2D) ???
it's simple for horizontal and verical lines... and even for straight lines with a known degree to the X axis (i think by controlling the speed for each motor and with some trigonometrics)... but what about circular like parts (not only circles, but any shape you can think of in 2D)...
thank you!